Flame resistant, epoxy resin impregnated, all glass cloth laminates have been used in the printed circuit industry to make NEMA (National Electrical Manufacturers Association) FR-4 high grade copper clad circuit boards. These high grade circuit boards must have excellent electrical properties, superior dimensional stability and mechanical strength, and outstanding chemical resistance. Laminates of this type are taught by Alvino et al., in U.S. Pat. No. 4,327,143; Pucci et al., in U.S. Pat. No. 4,343,731; and Pucci, in U.S. Pat. No. 4,311,753, most using tetrabromobisphenol A as flame retarding agent and a form of dicyandiamide as curing agent.
While these FR-4 grade laminates have excellent electrical and mechanical properties, they are expensive, and it is difficult to cold punch holes through them. These all woven, glass cloth laminates are typically drilled to provide through holes. Thus, the search continues for inexpensive electrical grade laminates having improved fabricating properties, along with improved thermal aging characteristics, higher bond strength and heat distortion temperatures, i.e., Tg values, and even better blister resistance; and for "B"-staged prepregs, useful for manufacturing such laminates, having improved shelf life, and low levels of ionic contamination.
In the area of flame resistant paper based laminates for copper clad circuit board application, Nichols, in U.S. Pat. No. 3,804,693, attempted to improve room temperature shearing, blanking, punching, and other fabricating properties. There, a paper web is first impregnated with a water soluble phenolic resin mixed with a solvent, such as methyl alcohol. Then, the wetted paper web is impregnated with an epoxy modified phenolic resin comprising an admixture of: oil and/or rosin modified phenolic resin; water soluble phenolic resin; and bisphenol A epoxy resin; to which is added a brominated or chlorinated flame resistant additive, such as chlorinated bisphenyl or tris (2,3-dibromopropyl) phosphate, mixed with antimony oxide. The phenolic resins taught are of the resole type, where the mole ratio of (phenol):(formaldehyde) is from (1):(1 to 1.5).
Claybaker, in U.S. Pat. Nos. 4,043,954 and 4,254,187, makes flame resistant laminates from paper web sheets impregnated with an epoxy modified phenolic resin mixture containing: tung oil modified phenolic resole resin; epoxy resin prepared from a tetrabromobisphenol A; a flame retardant mixture of triphenyl phosphate and antimony trioxide; barium hydrate; low viscosity, catalyzed phenol-formaldehyde resin to facilitate paper penetration; and solvent.
Lee and Neville, in the Handbook of Epoxy Resins, McGraw-Hill, 1967, ch. 11-13 to 11-16, teach a wide variety of curing agents for epoxies, including resole phenol-formaldehyde resins; and also solid, high molecular weight phenolic novolac resins, for use with high melting point bisphenol A epoxy resins in solvent solution, when catalyzed with benzyldimethylamine.
Union Carbide Corporation, in a brochure titled Flexol Plasticizer EPO, 1968, describes epoxidized soybean oil as a plasticizer designed especially for use in polyvinyl chloride homopolymer and copolymer resins, and methyl methacrylate and polystyrene resins.
Harrington, in U.S. Pat. No. 3,378,434, utilized flexibilizing agents selected from polyesters, including propylene glycol-adipic acid resins in brominated epoxyepichlorohydrin bisphenol A resin impregnants utilizing halogenated anhydride curing agents, for use in fire resistant, paper based laminates.
Flame resistant, punchable, composite laminates, made from a resin impregnated cellulose paper core bonded to resin impregnated woven glass cloth surface layers, are taught by Gause et al., in U.S. Pat. No. 3,895,158. There, resins comprising bisphenol A or novolac epoxy, in combination with chlorinated phenol or brominated epoxy flame retardants, and curing agents such as dicyandiamide, are used in the glass cloth layer. Liquid lubricating oils are excluded. The cellulose paper core is impregnated with various resins including the same epoxy resin used in the surface layer but with an anhydride curing agent.
Flame resistant, punchable composite laminates, made from a resin impregnated nonwoven glass fiber core bonded to resin impregnated woven glass cloth surface layers are taught by Benzinger in U.S. Pat. No. 3,617,613 with epoxy resins and other thermosetting resins disclosed as binders. Similar structures are disclosed by Zinbarg in U.S. Pat. No. 3,499,821 and by Zolg et al. in U.S. Pat. No. 3,393,117.
Additionally, Marchetti et al., in copending application Ser. No. 490,098, filed on Apr. 29, 1983, assigned to the assignee of this invention, discloses a variety of laminates wherein the epoxy resin is cured with a phenolic novolic oligomer.
The improvements described in the prior art do not, however, solve a cold punching halo effect problem around through holes that has recently developed in the field. As the electronics art has moved toward greater miniaturization, the density of components mounted on printed circuit boards has increased. As a consequence of the increased density of components, there is also an increased density of punched through holes. There has also been a movement to a more mechanized or automated wave soldering process for rapidly soldering the component leads to the circuit board pattern. These developments have resulted in an undesirable, high re-work (hand soldering) rate after the wave soldering operation, with a number of previously acceptable commercially available punchable composite laminates, which we believe to be attributable to a need for improved punched hole quality. It is apparent that there is a need for new and improved cold punchable, flame resistant electrical grade composite copper clad printed circuit boards.